🚀 go-pugleaf

In article <NEWTNews.820626346.32539.jrhodes@jrhodespc.nwest.mccaw.com>,
jrhodes@jrhodespc.nwest.mccaw.com wrote:

>In article <4c1q90$n37@mercury.near.net>, <fgoldstein@bbn.com> writes:
>> 
>> No, RB's "wet hay" is more accurate.  The 56k restriction comes about
>> because 
>> a) T1 carrier using old-fashioned AMI signaling can't allow the
>> all-zero octet to be transmitted, so they ALWAYS set it to 1 during a 

...

>Fred, can you help me understand what the yellow alarm on a T-1 for
>"15 zeroes" is? I thought an AMI T-1 could not take 15 consecutive
>zeroes before possibly slipping sync. It needs those 1's to make
>marks. Alternate Mark Inversion (AMI) means successive marks are
>different polarity, unless they are the same polarity, which is
>an AMI violation, which is used to mean something else. I don't 
>think a 1 is automatically inserted by the transmission gear as 
>you suggest in a), else there would be no need for a "15 zeroes"
>alarm. B8ZS within an ESF allows bi-polar violations within an 
>ESF frame to identify the all zeroes octet by inserting two marks
>with the same polarity within the octet. On non-ESF T-1s, a 
>bi-polar AMI violation is used to indicate the start of a new 
>193 bit non-ESF T-1 frame.
>
>Also, in b) aren't many of the channels completely inert with 
>regards to the "inband" robbed-bit signaling? If only three or 

...

My understanding:

The 15 zero limitation dates back quite a ways (40 years?) to when the
local clock recovery circuits weren't terribly stable timewise.  Now you
can push this a little harder.  However, almost all T1 transmission gear
will jam a bit to a one when it sees an all zero octet in a channel. On a
voice call, you really will never notice this happening, but on data... 
B8ZS was supposed to fix this 64 clear issue, but it has not been rolled
out on all facilities (some small problem with the money...)

Yellow alarm is used by one channel bank to inidicate to the other channel
bank that the first has lost frame synchronization (not bit sync.). 
Yellow alarm is transmitted by setting bit 2 on every channel to a one.

Actually the start of the frame is indicated by a specific bit pattern
appearing every 193 bits.  (that is, guess which bit is the framming bit,
and observe that bit and every 193 bits later and see if the bit pattern
matches... if not, go to the next bit).  The AMI/B8ZS line coding is LINE
level coding not frame level coding... this AMI or B8ZS stuff happens
INCLUDING the framing bit (and Fs bits too).

As to the last issue, when I fooled with this stuff, setting robbed bit
signaling on did so for every channel in the T-1.

Dave

-- 
Dave Gellerman                       dave_gellerman@newbridge.com
Newbridge Networks                       http://www.newbridge.com                  
Herndon VA USA                                       703 834-5313